generaldelta: initialize basecache properly

Previously basecache was incorrectly initialized before adding the first
revision from a changegroup. Basecache value influences when full revisions are
stored in revlog (when using generaldelta). As a result it was possible to
generate a generaldelta-revlog that could be bigger by arbitrary factor than its
non-generaldelta equivalent.

--HG--
branch : stable
help: use full name of extensions to look up them for keyword search

Before this patch, "hg help -k KEYWORD" fails, if there is the
extension of which name includes ".", because "extensions.load()"
invoked from "help.topicmatch()" fails to look such extension up, even
though it is already loaded in.

"help.topicmatch()" invokes "extensions.load()" with the name gotten
from "extensions.enabled()". The former expects full name of extension
(= key in '[extensions]' section), but the latter returns names
shortened by "split('.')[-1]". This difference causes failure of
looking extension up.

This patch adds "shortname" argument to "extensions.enabled()" to make
it return shortened names only if it is True. "help.topicmatch()"
turns it off to get full name of extensions.

Then, this patch shortens full name of extensions by "split('.')[-1]"
for showing them in the list of extensions.

Shortening is also applied on names gotten from
"extensions.disabled()" but harmless, because it returns only
extensions directly under "hgext" and their names should not include
".".

--HG--
branch : stable